The present invention relates to a process of producing spinel-type lithium manganate. More particularly it relates to a process for producing spinel-type lithium manganate which, when used as a cathode material of a nonaqueous secondary battery, suppresses dissolution of manganese therefrom thereby securing improved high-temperature characteristics of the battery such as high-temperature storage properties and high-temperature cycle characteristics.
With the recent rapid development of portable and wireless electronic equipment such as personal computers and telephones, the demand for secondary batteries as a driving power source has been increasing. In particular nonaqueous secondary batteries are expected for their smallest size and high energy density. Cathode active materials for nonaqueous secondary batteries meeting the demand include lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), lithium manganate (LiMn2O4), etc. Having an electrode potential of 4 V or higher with respect to lithium, these complex oxides are capable of providing batteries having a high energy density.
Of the above-described complex oxides, LiCoO2 and LiNiO2 have a theoretical capacity of about 280 mAb/g, while LiMn2O4 has a theoretical capacity as low as 148 mAh/g, but is deemed suited for use in electric vehicles and the like because of an abundant and inexpensive supply of manganese oxide as a raw material and freedom from such thermal instability in charging as observed with LiNiO2.
However, lithium manganate (LiMn2O4) is disadvantageous in that manganese dissolves out in high temperature to reduce the high-temperature battery performance, such as high-temperature storage properties and high-temperature cycle characteristics.
Accordingly, an object of the present invention is to provide a process for producing spinel-type lithium manganate which, when used as a cathode material of a nonaqueous secondary battery, suppresses dissolution of manganese therefrom thereby securing improved high-temperature characteristics of the battery such as high-temperature storage properties and high-temperature cycle characteristics and to provide a nonaqueous secondary battery using the cathode material.
Japanese Patent Laid-Open No. 139861/90 teaches that addition of a given amount of sodium to spinel-type lithium manganate brings about improvement on the room temperature cycle life. The publication describes a process comprising adding a sodium raw material to a manganese raw material and a lithium raw material and firing the mixture. Being inexpensive and abundant, electrolytic manganese dioxide is suitable as a manganese raw material for spinel-type lithium manganate. After electrolysis, electrolytic manganese dioxide is usually neutralized with ammonia for use in manganese dry batteries and with soda for use in alkali manganese batteries. It is known that soda-neutralized electrolytic manganese dioxide contains a small amount of residual sodium. The amount of the residual sodium depends on the neutralization conditions.
Having noted the neutralization conditions of electrolytic manganese dioxide, the present inventors have found that spinel-type lithium manganate obtained under specific neutralization conditions accomplishes the above object.
The present invention has been completed based on the above finding and provides a process of producing spinel-type lithium manganate which is characterized by comprising pulverizing electrodeposited manganese dioxide, neutralizing the powder with sodium hydroxide or sodium carbonate to a pH of 2 or higher, mixing the resulting electrolytic manganese dioxide with a lithium raw material, and firing the mixture.